The present invention relates to the removal of sulfur from a hydrocarbon feedstock. In another embodiment, the present invention relates to a reforming process using a highly sulfur sensitive catalyst which can be efficiently and effectively run for up to two years.
Generally, sulfur occurs in petroleum and syncrude stocks as hydrogen sulfide, organic sulfides, organic disulfides, mercaptans, also known as thiols, and aromatic ring compounds such as thiophene, benzothiophene and related compounds. The sulfur in aromatic sulfur-containing ring compounds will be herein referred to as "thiophene sulfur".
Conventionally, feeds with substantial amounts of sulfur, for example, those with more than 10 ppm sulfur, are hydrotreated with conventional hydrotreating catalysts under conventional conditions, thereby changing the form of most of the sulfur in the feed to hydrogen sulfide. Then, the hydrogen sulfide is removed by distillation, stripping or related techniques. Unfortunately, these techniques often leave some traces of sulfur in the feed, including thiophene sulfur, which is the most difficult type to convert.
Such hydrotreated naphtha feeds are frequently used as feeds for catalytic dehydrocyclization, also known as reforming. Catalytic reforming processes play an integral role in upgrading naphtha feedstocks to high octane gasoline blend stocks and for chemicals such as benzene, toluene and xylenes. These processes have become more important in recent years because of the increase in demand for low-lead and unleaded gasolines. However, some of the catalysts used in reforming are extremely sulfur sensitive, particularly those that contain zeolitic components. It is generally recognized, therefore, that the sulfur content of the feedstock must be minimized to prevent poisoning of such reforming catalysts.
One conventional method for removing residual hydrogen sulfide and mercaptan sulfur is the use of sulfur sorbents. See, for example, U.S. Pat. Nos. 4,204,947 and 4,163,708, the contents of which are hereby incorporated by reference. The concentration of sulfur in this form can be reduced to considerably less than 1 ppm by using the appropriate sorbents and conditions, but it has been found to be difficult to remove sulfur to less than 0.1 ppm, or to remove residual thiophene sulfur. See, for example, U.S. Pat. No. 4,179,361 the contents of which is hereby incorporated by reference, and particularly Example 1 of that patent. Very low space velocities are required to remove thiophene sulfur, requiring large reaction vessels filled with sorbent. Even with these precautions, traces of thiophene sulfur still can be found.
See also U.S. Pat. No. 4,456,527, the contents of which is hereby incorporated by reference, disclosing a hydrocarbon conversion process having a very high selectivity for dehydeocyclization. In one aspect of the disclosed process, a hydrocarbon feed is subjected to hydrotreating, and then the hydrocarbon feed is passed through a sulfur removal system which reduces the sulfur concentration of the hydrocarbon feed to below 500 ppb (0.5 ppm). The resulting hydrocarbon feed is then reformed.
Various possible sulfur removal systems are disclosed for reducing the sulfur concentration of the hydrocarbon feed to below 500 ppb. The various systems mentioned include
passing the hydrocarbon feed over a suitable metal or metal oxide, for example copper, on a suitable support, such as alumina or clay, at low temperatures in the range of 200.degree. F. to 400.degree. F. in the absence of hydrogen; or, PA1 passing a hydrocarbon feed, in the presence or absence of hydrogen, over a suitable metal or metal oxide, or combination thereof, on a suitable support at medium temperatures in the range of 400.degree. F. to 800.degree. F.; or, PA1 passing a hydrocarbon feed over a first reforming catalyst, followed by passing the effluent over a suitable metal or metal oxide on a suitable support at high temperatures in the range of 800.degree. F. to 1000.degree. F.; or PA1 passing a hydrocarbon feed over a suitable metal or metal oxide and a Group VIII metal on a suitable support at high temperatures in the range of 800.degree. F. to 1000.degree. F.
Attempts continue, however, to reduce the amount of sulfur contained in the hydrocarbon feeds so as to a permit a longer useful life for zeolitic catalysts. Once a sulfur sensitive zeolitic catalyst is poisoned, it is very difficult if not impossible to regenerate the catalyst. Therefore, due to the presence of expensive metals such as platinum in such catalysts, the longer the useful life of the catalyst the more practical the process employing such a zeolitic catalyst becomes.
Accordingly, in U.S. Pat. No. 4,925,549 there is disclosed a process for removing sulfur to less than 0.1 ppm (100 ppb) in an attempt to protect reforming catalysts which are sulfur sensitive. This patent, the contents of which is hereby incorporated by reference, discloses a method which comprises first contacting a feedstock with hydrogen under mild reforming conditions in the presence of a less sulfur sensitive reforming (or sulfur conversion) catalyst. This carries out some reforming reactions and also converts trace sulfur compounds to hydrogen sulfide. The effluent from the first step is then contacted with a solid sulfur sorbent to remove the H.sub.2 S and provide an effluent which contains less than 0.1 ppm sulfur. This low sulfur containing effluent can then be contacted with the highly selective reforming catalyst which is extremely sulfur sensitive.
While the state of the art has therefore progressed to protecting reforming catalysts which are sulfur sensitive to a large extent, greater protection is still desirable. Better catalyst stability than found in prior art processes using zeolitic catalysts is still an important objective of the art. The greater the stability of the catalyst, the longer the run length, which results in less down time and expense in regenerating or replacing the catalyst charge. The longer the run lengths, the more commercially practical the process. Without sulfur poisoning, it is believed that the practical useful life of a zeolitic catalyst is up to about two years. Therefore, a system which would permit a run length of up to about two years while using the highly preferred, but highly sulfur sensitive zeolitic catalysts would certainly be of a great practical advantage to the petroleum reforming industry.
Accordingly, it is an object of the present invention to provide a process which can remove substantially all sulfur, including thiophene sulfur, from a reforming feedstream.
Another object of the present invention is to provide a process which can efficiently reduce the amount of sulfur in a hydrocarbon feedstream to about 1 ppb or less.
Another object of the present invention is to integrate a sulfur removal system into a reforming process which would permit a practical useful life for the catalyst, e.g., of up to about two years.
These and other objects of the present invention will become apparent upon a review of the following specification, the drawing and the claims appended hereto.